1. Field of the Invention
An aspect of the present invention relates to a pulsed light generating method for generating a pulsed light by cutting out apart of a laser light outputted from a laser light source with an intensity modulation type electro optic modulator, and also relates to a pulse laser apparatus. The present invention also relates to laser systems such as an exposure apparatus and an inspection apparatus having such a pulse laser apparatus.
2. Description of Related Art
Pulse laser apparatuses are used as light sources for laser systems such as microscopes, shape measurement apparatuses, exposure apparatuses, and inspection apparatuses, for example. A wavelength of a pulsed light outputted by the pulse laser apparatus is set depending on uses and functions of a system into which the pulse laser apparatus is integrated. For example, a pulse laser apparatus that outputs a pulsed light having a wavelength of 193 nm, a pulse laser apparatus that outputs a pulsed light having a wavelength of 355 nm, and other pulse laser apparatuses are used in semiconductor exposure apparatuses and liquid crystal exposure apparatuses. A wavelength of a laser light generated by a laser light source, the presence or absence and the number of amplifiers, a combination of wavelength conversion optical elements in the case a wavelength conversion unit is provided, or the like are set depending on uses and functions of the laser system (see Japanese Laid-Open Patent Publication No. 2004-086193, for example).
One way of generating a pulsed light by a pulse laser apparatus is an approach of cutting out a part of a laser light outputted from a laser light source with an intensity modulation type electro optic modulator and outputting the part of the laser light. FIG. 16 illustrates a schematic configuration diagram of a pulse laser apparatus LS9 implementing this approach. The pulse laser apparatus LS9 is configured to include a laser light source 911 that outputs a laser light, and an intensity modulation type electro optic modulator (EOM) 912 that outputs a pulsed light by cutting out a part of the laser light outputted from the laser light source 911 (see Japanese Laid-Open Patent Publication No. 2013-004597, for example).
FIG. 17 illustrates an example of operating characteristics of an intensity modulation type electro optic modulator (hereinafter referred to as an EO intensity modulator) 912. The horizontal axis in FIG. 17 represents a voltage applied to the EO intensity modulator 912 and the vertical axis represents a transmittance of the EO intensity modulator 912 normalized between 0 and 1. In the EO intensity modulator 912 having the illustrated operating characteristics, a voltage V0 at which the transmittance is minimum is 2, 10, 18, . . . [V], and Vπ, the difference between the voltage V0 and the voltage which gives the maximum transmittance, is 4 [V]. In other words, the transmittance is 0 when the voltage applied to the EO intensity modulator 912 is V0 (2, 10, 18 . . . [V]), while the transmittance is 1 when the applied voltage is V0±Vπ (6, 14 . . . [V]).
Thus, the conventional pulse laser apparatus is configured to cut out a part of the laser light outputted from the laser light source 911 in order to output a pulsed light, by varying the voltage applied to tire EO intensity modulator 912 between V0 and V0±Vπ. FIG. 18 illustrates a timing chart of generating a pulsed light in a conventional pulse laser apparatus. In FIG. 18, chart (a) illustrates an ON/OFF state of the laser light incident onto the EO intensity modulator 912, chart (b) illustrates a voltage applied to the EO intensity modulator 912, and chart (c) illustrates an ON/OFF state of the light pulse outputted from the EO intensity modulator 912.
As illustrated, the voltage V0 is applied to the EO intensity modulator 912 in a time period when no pulsed light is outputted. When the pulsed light is outputted, an electrical pulse having an amplitude of Vπ with reference to the voltage V0 is applied to the EO intensity modulator 912. As a result, an pulsed light having an intensity amplitude corresponding to the maximum transmittance is outputted. A time width of the pulsed light obtained here is generally the same as a time width of the electrical pulse applied to the EO intensity modulator 912. In other words, in pulsed light generating methods in conventional pulse laser apparatuses, the following relationship holds: (electrical pulse waveform applied to EO intensity modulator 912)≈(pulsed light waveform outputted from pulse laser apparatus LS9). The amplitude of the electrical pulse may be set between V0 and V0+Vπ as appropriate, although the amplitude is here Vπ for the purpose of maximizing the intensity amplitude of the pulsed light.